Rechargeable metal-ion batteries, for example lithium ion batteries, are extensively used in portable electronic devices such as mobile telephones and laptops, and are finding increasing application in electric or hybrid electric vehicles.
Rechargeable metal ion batteries have an anode layer; a cathode layer capable of releasing and re-inserting metal ions; and an electrolyte between the anode and cathode layers. When the battery cell is fully charged, metal ions have been transported from the metal-ion-containing cathode layer via the electrolyte into the anode layer. In the case of a graphite-based anode layer of a lithium ion battery, the lithium reacts with the graphite to create the compound LixC6 (0<=x<=1). The graphite, being the electrochemically active material in the composite anode layer, has a maximum capacity of 372 mAh/g.
The use of a silicon-based active anode material, which may have a higher capacity than graphite, is also known. Silicon may be provided in the form of fibres.
WO 2009010758 discloses a process of forming silicon fibres by a first step of anisotropic etching of silicon particles to form a particle having a silicon core with silicon pillars extending from the core, and a second step of detaching the pillars from the pillared particle core by scraping, agitating or chemical etching of the pillared particle.
It will be appreciated that the yield of silicon fibres produced by this method as a percentage of the mass of the starting material is limited because the silicon of the starting material that is etched away in the first etching step cannot contribute to the mass of the silicon fibre product. It may also be the case that not all of the remaining silicon core can be recycled to produce further fibre so that this too cannot contribute to the mass of the fibre product. Furthermore, the silicon particles forming the starting material are not naturally available and must be manufactured using industrial processes that increase the cost and carbon footprint.
Jia et al, “Novel Three-Dimensional Mesoporous Silicon for High Power Lithium-Ion Battery Anode Material”, Adv. Energy Mater. 2011, 1, 1036-1039 and Chen et al, “Mesoporous Silicon Anodes Prepared by Magnesiothermic Reduction for Lithium Ion Batteries”, Journal of The Electrochemical Society, 158 (9) A1055-A1059 (2011) disclose formation of mesoporous silicon by magnesiothermic reduction of a silica template.
Yu et al, “Reversible Storage of Lithium in Silver-Coated Three-Dimensional Macroporous Silicon”, Adv, Mater, 2010, 22, 2247-2250, discloses magnesiothermic reduction of silica powder.
Richman et al, “Ordered Mesoporous Silicon through Magnesium Reduction of Polymer Templated Silica Thin Films”, Nano Lett., Vol. 8, No. 9, 2008, 3075-3079 a process of producing mesoporous silica (SiO2) thin films via evaporation induced self-assembly (EISA) using sol-gel silica precursors with a diblock copolymer template, followed by reduction of the silica to silicon by magnesium vapour.
It is an object of the invention to provide a method of forming elongate silicon-comprising structures.
It is a further objection of the invention to provide a high yielding method of forming structured particles containing elongate silicon-comprising structures as elements of the structured particles.
It is a further object of the invention to provide a sustainable method of forming elongate silicon-comprising structures on a large scale.
It is a further object of the invention to provide a method of forming elongate silicon-comprising structures with certain shape, form and dimensional characteristics that provide performance improvements in their application yet are not otherwise easily manufactured in bulk by other methods.